1. Field of the Invention
The invention relates to methods and apparatus for cameras sensitive to gamma radiation.
2. Background
Gamma radiation is produced by the nuclei of certain radioactive atoms during annihilation of positrons by electrons. Cameras sensitive to gamma radiation are used to identify and locate sources of gamma radiation in the vicinity of the camera.
Gamma radiation may be produced in a human or animal by injecting certain isotopes (radionuclides) which decay by emission of positrons. Cameras with gamma radiation detectors arranged in circular or cylindrical arrays may be placed around a human or animal subject to locate areas of concentration of the injected isotopes within the animal or human body. This is positron emission tomography (PET). Analogous equipment and procedures can be used to locate isotopes within inanimate objects when the isotopes decay by emission of positrons.
Throughout this application, "PET camera" and similar terms will be understood to include cameras sensitive to gamma radiation from any source within the object field of the camera.
PET Applications in Oncology
Oncology is becoming a very important area of clinical PET application. The strengths of positron imaging are mainly its group of metabolically important radionuclides, cancer specific chemistry, and the much higher gamma-detection efficiency of coincidence collimation as compared to prior art gamma-cameras. PET's tomographic reconstruction capability has also been important clinically in describing the location of PET tracer radiation sources within the body.
Significance of PET in Breast Cancer
Statistics show that one in nine American women will be diagnosed with breast cancer. The disease kills 45,000 women each year and is the leading cause of death for women 35-50; the reported incidence of breast cancer increased by 32% from 1982 to 1987. Increased awareness has caused breast cancer to become a high profile disease in the medical community, in the news media, and with women's groups, all of whom call for better early diagnosis and treatment. There is an urgent need for improved early diagnosis in women under 35 years old.
Breast Cancer Detection in Younger Women
X-ray mammography, which images tissue density differences, is relatively ineffective for cancer diagnosis in younger women (i.e., women less than 50 years old, as in a Canadian study of 90,000 women reported in 1993). Normal breast tissue in many such women has nearly the same density as cancer (&lt;1% difference). Furthermore, differentiating benign from malignant lesions is difficult in mammograms. Although malignant and normal cells are easily distinguished physiologically, such differences are not apparent in mammograms because they are metabolic in nature and have little effect on tissue density to x-rays. Thus, the large 1993 Canadian National Breast Screening Study (NBSS) noted above supported a conclusion that screening x-ray mammography confers no survival benefit in women ages 40-49.
In contrast, higher metabolic rates associated with the 6-13 times higher proliferation rates characteristic of malignant cells compared to normal cells are easily detected by PET with fluorine-18-deoxyglucose (FDG) and thymidine tracers. Additionally, the relatively higher metabolic rate of malignant cells is even more pronounced in younger women because breast cancer generally grows faster in such women than in older women. Hence, the natural advantage of PET over x-ray mammography in breast cancer detection is even further enhanced in younger women, where x-ray mammography is least effective. The ability to detect cancer through metabolic measurements also makes PET useful in assessing the effectiveness of chemotherapy much sooner than anatomic imaging devices, e.g., x-ray, computerized x-ray tomography, or magnetic resonance imaging.
Another advantage of PET over x-ray mammography is related to the specificity of PET screening results. About 80% of the masses detected by x-ray mammography are false positives, but cancer cannot be ruled out in these cases except by biopsy. Thus, about 500,000 fine needle biopsies (guided by ultra-sound or x-ray computed tomography) are performed annually at a cost of $1000 to $2000 per biopsy.
The diagnostic information from a properly placed biopsy is very valuable, but biopsy would not be available for the (approximately 10% of) actual tumors which are missed in routine x-ray mammography, or for the tumors which escape detection because of the x-ray shielding effect of silicone breast implants. On the other hand, PET scans with a resolution of 3-4 mm which could be administered for about $1000 to $1500 would be a substantial aid in diagnostic screening for breast cancer.
Opposing these reasons for increasing PET usage are the high price of PET cameras (ca. $2,500,000), high camera maintenance costs (ca. $250,000/year), and large space requirements for conventional PET cameras. These cost factors affect how PET may be used in screening and diagnostic studies, and the same factors would be sensitive to reductions in the size and cost of the camera itself.
Another problem in using the conventional ring form PET camera is the small axial field-of-view (typically 10-15 cm). If one wants to find cancer and its metastases throughout the body, many scans may have to be made in 10-15 cm steps (at 10-20 minutes per step). Because of the short decay half-life of most positron isotopes, such time-consuming stepping through the whole body is often difficult. To illustrate, the half-lives of commonly used radionuclides are as follows: 0-15, 2 minutes; N-13, 10 minutes; C-11, 20 minutes; F-18, 110 minutes. Even if imaging can be accomplished with these tracers, it is relatively long and the patient throughput per day is limited. Further, imaging in sequential steps only creates a snap-shot in time of a 10-15 cm cross-section; this means that tracer-uptake as a function of time for any given cross-section cannot be imaged. Hence, cancer detection is compromised because the dynamic uptake of positron tracer over time is often important for cancer imaging.